In order to reduce the harmful effects of lead in lead brass faucets, relevant researchers from both China and overseas have studied the corrosion mechanism of brass caused by drinking water and the effect on the corrosion resistance by adding alloying elements to brass. Various measures have been taken to improve the corrosion resistance of brass, such as adding tin, nickel or other alloying elements, removing the soluble lead, or inhibiting the leaching of lead and so on. However, since lead is an alloy element of such brass and always exists in brass, the above methods can only reduce the side effects of lead to a certain extent and cannot fundamentally eliminate the harm of lead. In view of this, there is an important issue to be solved in the industry in finding a new alternative material for a copper alloy faucet.
In recent years, a lot of research on lead-free easy cutting brass has been conducted both in China and overseas, and some achievements have already been achieved, mainly utilizing silicon, bismuth, magnesium, antimony and graphite instead of lead. In particular, silicon brass has excellent performance of casting, thermal processing, welding, resistance to dezincification, and stress corrosion, coupled with the low-cost advantage of silicon, so the position of brass material in the green and environmentally friendly lead-free easy cutting industry is particularly prominent. Among them, the patent reference “Easily processed silicon brass alloy and preparation method thereof” filed by the Jomoo Kitchen & Bathroom Appliances Co., Ltd. (publication No. CN 104651660 A, Reference Document 1) discloses that the composition of the alloy includes: 60-63 wt % Cu, 0.50-0.90 wt % Si, 0.50-0.80 wt % Al, 0.10-0.20 wt % Pb, less than 0.3 wt % other additional trace elements, with the balance being Zn and unavoidable impurities. However, the silicon brass alloy still contains the constituent of Pb. By calculating the zinc equivalent of the example in this patent reference, the structure of such alloys should consist of two phases of α and β.
The patent reference “Lead-free silicon brass alloy and preparation method” filed by the Jiuxing Holding Group (publication No. CN 103725922 A, Reference Document 2) discloses the composition of the alloy includes: 59-63 wt % Cu, 1-1.5 wt % Si, 0.001-0.05 wt % Al, 0.001-0.01 wt % B, 0.1-0.5 wt % Fe, 0.1-0.2 wt % Mn, 0.1-0.15 wt % Sn, 0.05-0.5 wt % P, 0.01-0.07 wt % rare earth element RE, with the balance being zinc and unavoidable impurities. By calculating the zinc equivalent of the example in this patent reference, the structure of such alloys should consist of two phases of α and β. However, the tensile strength of 430 MPa-460 MPa can be further increased to some extent, and the dezincification layer thickness of 210 μm can also be further reduced to some extent, so as to obtain more excellent comprehensive performance.
In addition, although the above patent references disclose the specific composition range of the alloy, the design principles and phase composition are not specified. In fact, the design principles and the phase compositions of the alloy greatly affect the tensile strength, the corrosion resistance, the cutting performance, and other comprehensive performance of the copper alloy.
The study on α and β biphasic brass, such as HPb59-1 lead brass, shows that the strength and hardness of β phase (CuZn-based solid solution) are higher than those of a phase (solid solution of Zn dissolved in Cu), but the β phase can be processed in hot and cold pressure and has better plasticity especially under hot processing conditions. However the γ phase (the solid solution based on an electronic compound Cu5Zn8) is different in that it is a hard brittle phase and is distributed like stars in the matrix in a casting state, which brings negative effects on the mechanical processing performance and service performance. Therefore, if a brass alloy had a β phase matrix where tiny dot-like γ phase was uniformly distributed, which played the role of breaking the chip in the cutting, the brass alloy would have similar cutting performance to lead brass. The key to realizing the idea is to design an appropriate zinc equivalent, so that the alloy consists of two phases, β and γ, and the γ phase is distributed, in a tiny dot-like and uniform dispersion manner, in the β phase matrix after a modification treatment.
According to the studies on brass, zinc equivalent should be at least 48 wt % or more if there is a γ phase generated in the alloy. Correspondingly, for a multi-component copper alloy, the necessary condition for the formation of γ phase is that the zinc equivalent of the alloy must be greater than 48 wt %. However, a zinc equivalent that is too high will result in the decrease of the plasticity of the alloy and seriously affect the cutting performance.
The formula for calculating the zinc equivalent is:
            X      ⁢                          ⁢              (        %        )              =                                        Cz            n                    +                      ∑                                          C                i                            ⁢                              K                i                                                                          Cz            n                    +                      C            Cu                    +                      ∑                                          C                i                            ⁢                              K                i                                                        ×      100      ⁢      %        ,wherein X is the zinc equivalent of complex brass after adding the alloying elements; CZn is the actual zinc content added to the alloy; CCu is the pure copper content actually added to the alloy; ΣCiKi is the product sum of all alloying elements contents Ci added to the alloy and the respective zinc equivalent values (zinc equivalents) Ki of the added alloying elements. Among them, the main regulating elements of the zinc equivalent of the brass alloy are silicon and aluminum, and their zinc equivalents are 10 and 6, respectively. Therefore, the zinc equivalent of the alloy can be regulated by the reasonable regulation of the contents of silicon and aluminum, and then the phase composition and the comprehensive performance of the alloy can be controlled.
In view of this, if a copper alloy composed of two phases, β and γ, were obtained by reasonable regulation of zinc equivalent and the γ phase was distributed in a tiny dot-like and uniform dispersion manner in the β phase matrix after a modification treatment, the lead-free copper alloy with excellent comprehensive performance of tensile strength, corrosion resistance, cutting, and the like would be produced to replace the lead brass material commonly used in the industry, which has an important theoretical and engineering significance.